Semiconductor devices are used in a wide variety of applications. Silicon has been used as the substrate material of choice due to cost and availability of single crystal silicon wafers. Driven by the microelectronic industry silicon wafer production has enabled the economical use of larger wafers with greater than 12 inch diameter. The low cost of silicon allows thick (1-2 mm) single crystal silicon wafers to be used without a secondary substrate for handling and processing. This enables the formation of large area die, which can be processed without the need for transfer substrates or wafer bonding. Additionally, the properties of silicon permit wafers to be doped so that 3 dimensional devices (via planar processing) can be created taking advantage of the wafer conductivity. For some devices it is desirable to utilize thinning techniques to reduce the overall thickness of the silicon to tens of microns to improve thermal performance. Although silicon has been the dominant material for most microelectronic applications there are other materials, which have desirable properties and advantages over silicon in the areas optoelectronics, solar cells and power devices. However, heretofore there has not been an economical solution to using these materials without growing them or attaching them to secondary substrates.
Silicon also has limitations with regard to operating temperature as well as other critical material parameters; this limits silicon's usefulness in optical, power and high-frequency applications. Nitride alloys and oxide alloys have several properties, which are superior to silicon ranging from higher thermal conductivity to biocompatibility. Unfortunately, nitrides are not available in wafer form at a reasonable price or reasonable quality. Even if such wafers were to become available, costly growth, dicing, and thinning techniques would be required to create useful devices. In most cases, devices with an overall thickness between 20 and 150 microns are desired from a thermal impedance, packaging, and optical efficiency standpoint. As such, film based processing offers several advantage over bulk wafer approaches. As the solar industry has discovered film and foil based processing is much more cost effective than wafer based approaches if high resolution lithography is not required. The need exists for cost effective thin nitride films, but also for the ability to process both sides of these films for the formation of waveguides, edge emitters, and symmetric device structures.
In copending patent applications, it has been shown that stacked LEDs, solar cells, and other optoelectronic devices based on freestanding nitride films offer significant advantages from the standpoint of current spreading, functionality, color combining, and peak drive levels. Nitride alloys and zinc oxide alloys offer a unique set of properties with regard to optical, electronic and optoelectronic devices. Further, it has been shown that freestanding nitride films, which exhibit alpha less than 1 cm-1 throughout the visible spectrum, can be created with thicknesses ranging from 20 microns to over 100 microns, which also exhibit resistivities less the 0.05 ohm cm and a thermal conductivity approaching 200 W/m/K with sufficient areas to enable device fabrication. This unique combination of properties at a thickness and area sufficient for handling and subsequent processing permits unique processing techniques including dual sided processing.
As disclosed previously, the use of freestanding nitride films containing MQW structures can be used to create isotropic light sources and the use of stacked freestanding nitride films with different MQW structures can be used to create isotropic white light sources. The combination of these devices with solid wavelength conversion materials has also been disclosed in a pending patent application. However to fully take advantage of these unique attributes, the need exists for low cost processes and materials, which can be used to create devices based on freestanding nitride films.
In particular, the need exists for techniques whereby processing including but not limited to epitaxial growth, deposition, laser patterning, printing, and various interconnect means can be done on one or both sides of freestanding nitride films for the formation of 3 dimensional devices and dual side processes. Zinc oxide alloys can be used to create 3 dimensional devices based on similar material properties to nitrides. While native ZnO wafers have been fabricated, their cost is restrictive and they lack a suitable/stable p-dopant which limit their applications. The combination of nitride alloys and zinc oxide alloys can be used to create a wide variety of high performance electrical and optoelectronic devices. Therefore there is a need for economical processes and means to create 3 dimensional devices based on nitride alloys and/or zinc oxide alloys and combinations of the two materials sets. In this manner, a wide range of hybrid devices can be realized.
While wafers offer significant advantages to high-resolution circuitry as used in microprocessors and memory devices, which require up to 50 masking steps, many applications do not require high-resolution lithography steps. In some cases, processing round wafers is actually a disadvantage or limitation to be overcome. As an example, solar cells are made in ribbon and large area formats. Alternately, liquid crystal displays contain semiconducting active-matrix backplanes are grown on large glass plates that measure four to 6 feet in dimension. Unlike the semiconductor industry, the thick film industry tends to use square substrates, which are more compatible with the printing techniques typically used to form patterned conductors and dielectrics. Square, ribbon and tape based substrates have less edge loss than yields on round wafers.
In addition, the formation of non-circular devices offers several advantages with regard to device performance. There is the potential to reduce stress and enhance extraction efficiency where the edges of the devices are aligned to natural cleavage planes. Triangular shaped die can be effectively utilized to form recycling light cavities, which can enhance radiance to light emitting diode light sources. In copending patent applications methods of forming large area nitride layers based on the removal of thick HVPE epitaxial layers from sapphire substrates are shown. The resulting freestanding foil substrates enable increased flexibility in packaging and device design. The need therefore exists for methods and articles, which take advantage of these freestanding nitride films and provide techniques for processing into devices, which take advantage of the material, geometry, thermal mass, and flexibility of these freestanding nitride films.
These freestanding films offer several advantages over nitride layers which are transferred to a secondary substrate via waferbonding techniques, and over films which remain on their growth substrate like sapphire or silicon carbide, and over diced and polished nitride wafers.
In the case of waferbonded films, stresses created during the original growth process are transferred via the wafer bonding process. Also the resulting structure suffers from poor thermal performance, thermal expansion mismatch which limits operating range. Typically the these nitride layers are thin (˜3 microns) which can impact packaging processes such as wirebonding to electrical contacts on the device. This is due to the fragile nature of the thin nitride film, requiring reduced bonding forces to prevent cracking of the nitride layer. This lowers yield in final device fabrication. The lower permissible operating temperature range due to the use of secondary substrates used in waferbonding not only limits device operation but also prevents the use of robust materials such as glass frits and fired contacts that can enhance packaging reliability but require high temperature processing.
With these prior art methods, nitride films left on their growth substrate must be thin to prevent bowing either at room temperature or at high temperature growth conditions. The low thermal conductivity of sapphire in particular limits device performance and also typically requires wafer thinning processes, which increase costs. Also, the inability to create a vertical structure for non-conductive growth substrates like sapphire limit the ability to form vertical structures or stack nitride films, which can result in new and novel devices.
Another prior art technique to form freestanding nitride substrates is to grow thick nitride layers on secondary substrates, followed by dicing and polishing to eliminate the secondary substrate. This has proven to be a very costly process and by necessity has size limitations. Further, defects introduced via dicing and polishing and the inclusion of stresses due to the dicing processes limit the yield and viability of this approach. In these prior art techniques dual sided processing is impossible or very difficult. However, dual sided processing could offer unique advantages and lead to new types of devices.
It would be desirable to have the ability to grow a variety of structures on a wafer level and then form a freestanding nitride film, which can be further processed on one or both sides. This could offer significant packaging and device flexibility. Alternately, the ability to grow directly on freestanding nitride films could offer many benefits including but not limited to; lower stress growth, dual side processing, low thermal mass and flexible substrates.
In copending patent applications we have shown where freestanding thick (15 to 150 micron thick) nitride layers can be formed with sufficient area and low cost. These layers have an epitaxial ready surface, can be stacked, and can be stress relieved either by allowing the films to bow or be subjected to high temperature annealing processes, once free of the growth substrate. In addition, features and additional layers can be added to one or both sides of the freestanding films. These freestanding nitride and oxide films can be processed on both sides at temperatures over 700 degrees C. enabling the use of thick film processes developed by the solar industry. The use of high temperature thick film processes to form LEDs, diodes, optoelectronic devices, Mems, solar cells and other semiconducting devices could benefit by dual sided processing. In addition, it would be possible to take advantage of the low thermal mass of thin freestanding nitride foils to enhance the epitaxial device growth process.